I had exact same issue, so did others. I have 8 core i7 xeon 16GB and 6GB NVidio geoforce.
I started a thread below (x plane v slow after update) in which there are some suggestions incl fiddling with anti-aliasing, tried them, tried disabling plugins, no joy, checked all drivers up to date. Did a data dump and max 14 fps with all sliders to low sitting on runway. My CPU only using max 20% but cpu and gpu times are similar at .09 down to .07 sec each per frame.
Tried to load xplane 10 to see if that works ok and now they seem to have disabled key so it won't run xplane 10 also!
I tried opening a few windows over xplane and there's oodles of unused computing power - other applications run happily over th front with xplane in the background and xplane taking up 20% most cpu, others a bit, still 60+% cpu idle, 50% ram unused, the disk is SSD so no obvious bottlenecks
Definitely a bug in the latest upgrade.
Hopefully we can consolidate threads and someone find a fix.
Meanwhile, does anyone know how to downgrade to xplane 11.x whichever it was that last worked ok?